## Implementing an external ME+PS combination scheme and interfacing this plugin with Pythia

For experts and developers of new matching/merging schemes, Pythia also offers the possibility to completely replace its internal merging machinery with a user-defined plugin code (much in the same way that parton shower plugins (cf. Implement New Showers) are possible). This allows for maximum flexibility while still benefiting from the full Pythia event generation machinery. Note that the ME+PS merging with the VINCIA and DIRE shower plugins make use of this flexibility, and might thus provide helpful clarifications.

Of course, implementing your own, new matching/merging scheme is a non-trivial task, and comprehensive guidelines on how to proceed are impossible to set. However, it is important that an external matching/merging plugin interfaces to Pythia in a simple and well-defined manner. Here, we will document which C++ functions are necessary to be able to use an external matching/merging (MM) plugin within Pythia.

To understand how to design a MM plugin for Pythia, it is useful to review how Pythia's internal merging machinery is structured. The interaction between the core Pythia and the merging code is governed by the `Merging` and `MergingHooks` classes. Note that for moderately complex requirements, it may be sufficient to only replace Pythia's instance of `MergingHooks` with a pointer to an external class (cf. CKKW-L merging). The latter two classes are supplemented with the helper classes `History` and `HardProcess`. The latter gathers information on the (user-supplied information about the) hard core scattering process to which hard jets are added by ME+PS merging. It is only used as a helper to the `MergingHooks` class. The `History` class contains the implementation of all internal (LO or NLO) merging schemes. The `Merging` class acts as a bridge between the implementation in the `History` class and the rest of the Pythia code.

To implement an external MM plugin, you will have to write classes that derive from the `Merging`, `MergingHooks` and `HardProcess` classes of Pythia. For special cases, it might also be permissible to only implement a replacement of the `Merging` class, while still using Pythia's implementation of the other two classes.

The external MM plugin can then be transferred to and used by Pythia much in the same way as `UserHooks` classes or shower plugins. More concretely, an external MM code will be used if a pointer to an instance of the external classes is transferred to Pythia via the methods

The option to only use a user-defined `MergingHooks` instance is already documented in the item CKKW-L merging and will not be discussed further. We will now focus on how to implement external `Merging`, `MergingHooks` and `HardProcess` classes that can be used as a complete replacement of the Pythia methods.

Let us assume that you want to create a class of type `MyMerging`, and you call its instance `myMerging`. For this external ME+PS merging class to be interfaced to Pythia, the class needs to inherit from the `Pythia8::Merging` base class. It is further necessary to define the following functions that serve as interface to Pythia:

A destructor for your ME+PS class. If not defined, the base class's empty destructor will be used.
A method that is used to initialize your merging class. Pythia will call this function during its initialization and after all pointers to internal classes (e.g. to instances of the `Info` and `ParticleData` classes) have been set up.
This function can be used to collect and print merging information at the end of the event generation. Pythia will call this function in the execution of a `Pythia::stat()` call.
This function should be the main interface of Pythia to the MM plugin. Pythia will execute this function once the partonic (fixed-order) scattering has been constructed (or read from LHEF). The partonic scattering is transferred via the `process` argument. The external MM plugin should then, based on the `process`, implement the matching/merging strategy. It is permissible that this function changes `process`. In this case, Pythia will continue the event generation with the changed `process` as starting point. The return value of the function steers how Pythia should proceed after the function execution. The following return values are supported:
- -1 : Reject the event and exit the generation/processing of the current event
- 0: Reject the event but continue with the generation/processing of the current event.
- 1: Keep the event but continue with the generation/processing of the current event.
- 2: Reject the event but continue with the generation/processing of the current event. However, re-evaluate resonance decays before any other event generation step. This option can be necessary if the merging code removes or changes resonant particles from `process`.

Note that because this function is the main interface between the MM plugin and Pythia, it is necessary to use this function to set up all the information that you might later need (merging weights, particle counters, etc) in this call already.

For more details on how to design your `MyMerging` class, and to understand the interface to Pythia, studying Pythia's internal code is unavoidable. Each potential developer of a MM plugin should do so.

The other main ingredient of the interface to MM plugins is a new implementation of the `MergingHooks` class. Let us assume that you want to create a class of type `MyMergingHooks`, and you call its instance `myMergingHooks`. For this class to be interfaced to Pythia, it will need to inherit from the `Pythia8::MergingHooks` base class.

A destructor for your MergingHooks class. If not defined, the base class's empty destructor will be used.
A method that is used to initialize your `MyMergingHooks` class. Pythia will call this function during its initialization and after all pointers to internal classes (e.g. to instances of the `Info` and `ParticleData` classes) have been set up.
This function will be used to tell Pythia if a CKKW-L-style event veto after the first parton shower emission should be checked. If so, the function should return true, and false otherwise.
This function will be used to implement the check of a CKKW-L-style event veto after the first parton shower emission, i.e. to check if the first parton shower emission is above the merging scale. If the input event `event` after emission should be kept, then false should be returned. If you want instead to veto the event and continue with a completely now hard scattering event, true should be returned.
This function will be used to tell Pythia if a veto of emissions should potentially be applied.
This function will be used to implement the check if shower emissions should be discarded, as e.g. necessary in UMEPS or UNLOPS merging. You can study the input event `event` after emission, and return true if the emission is valid, and false if you want to reject the emission. Note that this veto does not lead to event rejections, only in potentially removing certain emissions during shower evolution.
This function allows to set the starting scales for timelike and spacelike showering as well as multiparton interactions. It is thus necessary to properly start trial showers (that generate necessary no-emission probabilities), and for setting the correct starting conditions for parton showering of accepted (non-zero weight) events. The input `event` gives the hard process before showers and MPI are attempted. If `isTrial=true`, this means that the function is currently called from within a trial shower object (to produce no-emission probabilities). If `doMergeFirstEmm=true`, then the function is called to set starting conditions for the shower evolution of an (accepted) event. The double arguments `pTscaleIn`, `pTmaxFSRIn`, `pTmaxISRIn` and `pTmaxMPIIn` are tentative values for the starting scales of FSR, ISR and MPI. The function may overwrite these with the desired values. Similarly, `limitPTmaxFSRin`, `limitPTmaxFSRin` and `limitPTmaxMPIin` inform Pythia if the phase space for FSR/ISR/MPI is restricted (true) or unrestricted (false). Again, the tentative values can be overwritten.

The `MergingHooks` base class allows for further virtual functions that are not directly called by Pythia, and are hence not necessary to define. Th usage of these functions within Pythia's `Merging` and `History` classes is documented in CKKW-L merging. The additional (optional) virtual functions are:

The internal implementation of `MergingHooks` in Pythia heavily relies on the `HardProcess` helper class. It is in principle not necessary to follow the same strategy when implementing a derived `MyMergingHooks` class. However, to benefit from the Pythia implementation, and to allow for a structure similar to the internal code also for an external MM plugin, it is also possible to effectively replace (in the `MergingHooks` class) the pointer to an instance of `HardProcess` with a pointer to an external implementation. Let us assume that you want to create a class of type `MyHardProcess`, and you call its instance `myHardProcess`. For this class to be interfaced to `MergingHooks` (or the derived `MyMergingHooks` class), it will need to inherit from the `Pythia8::HardProcess` base class.

A destructor for your HardProcess class. If not defined, the base class's empty destructor will be used.
This function can be used to initialize the instance of your HardProcess class. In the internal Pythia implementation, this acts as a wrapper around the next function.
This function will use the string argument to set up the hard process bookkeeping, e.g. how many incoming/outgoing particles of which flavour are contained in the core (lowest multiplicity) scattering process.
This function studies the input event and book-keeps the particles that may be considered as part of the core scattering process. For this, it may use the four next functions.
This function uses the input vectors of positions of particles in the input event to decide if the particle with `iPos` could be member of the core scattering. If the particle with position `iPos` cannot be part of the core scattering (e.g. because it is a final state parton, while the core scattering contains final state leptons only), then the function should return false. Else, return true to allow this candidate. Note that it might be possible to find multiple equally good core scattering candidates. In this case, all candidates should be found (with the `findOtherCandidates` function), and can be potentially be replaced (with `exchangeCandidates`).
This function may be used to check if the particle with position `iPos` in the input event should be considered an outgoing particle of the core scattering.
The argument `iPos` specifies the position of a particle in the input event which is tagged as part of the core scattering. This function may be used to check the role of `iPos` as core scattering member may be filled by another particle in the event record. If so, and if `doReplace=true`, then `iPos` will no longer be book-kept as part of the core scattering. An example where this functionality is helpful is if the input event is g g -> b b~ b b~, and the core scattering is g g -> b b~. Not swapping the hard process candidates could in this case mean that not all parton shower histories can be found. The function should return false if no replacements can be found, and true otherwise.
This function implements the replacement of a list of core scattering candidates by another list of candidates.